1. Field of the Invention
The present invention relates to a differential limiting torque control system for an automotive vehicle employing both a limited slip differential generally abbreviated as a "LSD" and an anti-skid brake control system generally abbreviated as an "ABS", in which a differential limiting torque of the LSD is electronically controlled by adjusting an engaging force of a clutch device operably arranged between a differential case and differential side gears, based on input information representative of an operative state of the ABS.
2. Description of the Prior Art
One such prior art differential limiting torque control system employed in an automotive vehicle with an ABS has been disclosed in Japanese Patent Provisional Publication (Tokkai Showa) No. 62-110530. In the event that the differential limiting torque is acting between the right and left drive road wheels during operation of the ABS, the anti-skid control tends to be affected by unpredicated braking torque which acts on the drive road wheel and is created by the differential limiting operation of the LSD. The unpredicated braking force would increase the possibility of the drive road wheel-lock. In order to avoid the interference between the anti-skid brake control and the differential limiting torque control, the above-mentioned conventional differential limiting control system is so designed as to completely release the differential limiting torque applied to the clutch device of the LSD in case that the anti-skid brake control and the differential limiting torque control are both executed concurrently with each other. However, the conventional differential limiting torque control system suffers from the drawback that the vehicle experiences a rapid change in yawing moment about the center of gravity thereof owing to the driving torque difference between the right and left drive wheels, in case that the differential limiting torque is rapidly adjusted to zero concurrently with activation of the ABS (shift to the operative state of the ABS), during moderate or gentle braking on turns with a high-level lateral acceleration exerted on the vehicle body on high frictional roads having a high friction coefficient namely dry pavements, or during gentle braking on turns on a split-.mu. road wherein one of the right and left drive wheels lies on a high-frictional road surface such as a dry pavement and the other drive wheel lies on a low-frictional road surface such as a muddy road, wet or icy roads. The rapid change in yawing moment would result in unstable behavior of the vehicle body. As is well known, the differential limiting torque control means a control of the yawing moment about the center of gravity of the vehicle body. The previously-noted rapid change in yawing moment deteriorates stability and controllability of the vehicle during braking deceleration on turns. In more recent years, there have been proposed and developed technologies for avoidance of the rapid change in differential limiting torque during operation of the ABS on low-friction roads such as snow or icy roads. For instance, Japanese Patent Second Publication (Tokko Heisei) No. 4-72731 discloses a slip controlling brake system for a four-wheel drive vehicle with a differential limiting device, in which the differential limiting operation is partly or completely released only during operation of the ABS. Such partial release could merely hold the differential limiting torque applied to the differential just before the ABS is switched to an operative state. The partially released differential limiting device cannot always produce a yawing moment enough to compensate the vehicle behavior to understeer positively. For example, assuming that the differential limiting torque held through the partial release is insufficient when the ABS is conditioned in the operative state during braking on turns with a high lateral acceleration on high-friction roads, there is a possibility of unstable vehicle behavior. That is, the insufficient differential limiting torque cannot ensure a stability of the vehicle during braking operation.